In the field of wireless communication systems based on the multiple-input multiple-output ultra-wide band (MIMO UWB) technology, signals can be sent with a ratio between the bandwidth and the center frequency that is greater than 20% or with a passband greater than 500 MHz. Terminals equipped with a number of antennas are capable of handling multimedia services, in telecommunication networks of the personal network type or very high speed (typically measured in hundreds of Mbits/s) wireless local area network type.
The description that follows is based on a communication system with P sending antennas and Q receiving antennas, P and Q being strictly positive integers.
UWB communications operating in pulse mode involve transmitting pulses of short duration (around a nanosecond). In most cases, the information is encoded via the position of these pulses (pulse position modulation, PPM) and/or via the amplitude of these pulses (pulse amplitude modulation, PAM).
The pulse-type UWB signals are detected by receivers belonging to two categories:
1) Rake-type receivers in which, on L parallel channels, corresponding to a Rake of order L, the received signal is correlated with L delayed and appropriately weighted versions of the pulse shape, the knowledge of these delays and amplitudes being acquired during a training phase; and
2) correlator-type receivers in which, to detect the symbols sent, the received signal is correlated with a reference signal, this reference signal being constructed during the channel estimation phase.
The inventive subject matter disclosed herein applies to the first category of receivers above.
One way of increasing the capacity and enhancing the performance of UWB communication systems is to apply multiple-input multiple-output (MIMO) processing techniques. The systems using such techniques are classified in two categories:
1) coded systems, where a space-time coding of a data stream is used to exploit the transmission diversity and enhance performance, but where the redundancy introduced by the code reduces the throughput of the sender; and
2) uncoded systems, applying a space multiplexing, where the data streams on the sending antennas are totally independent, which makes it possible to increase the throughput of the sender. However, numerous disturbances are observed between the signals sent in parallel.
The inventive subject matter disclosed herein belongs to the second category of systems described above.
Two MIMO UWB sender architectures are known in particular.
The first is introduced by M. WEISENHORN et al. in a paper entitled “Performance of binary antipodal signaling over the indoor UWB MIMO channel”, published for the IEEE conference on communications, vol. 4, pages 2872 to 2878, May 2003, and by W. SIRIWONGPAIRAT et al. in a paper entitled “On the performance evaluation of TH and DS UWB MIMO systems”, published for the IEEE conference on wireless communications and networking, vol. 3, pages 1800 to 1805, 2004, and is illustrated in FIG. 1.
Such a UWB sender has P sending antennas and uses space multiplexing. A training phase, consisting in sending sequences (represented by dashes in FIG. 1) known to the receiver, precedes the data transmission. All the antennas send the same pulse shape w1(t) which corresponds to the nth derivative of a Gaussian.
The throughput is increased by a factor equal to the number of sending antennas, but this simple approach limits the performance of the system.
A second UWB sender architecture is outlined by E. BACCARELLI et al. in a paper entitled “A simple multi-antenna transceiver for ultra wide band based 4GWLANs”, published in IEEE WCNC, vol. 3, pages 1782 to 1787, March 2004, and by E. BACCARELLI et al. in a paper entitled “A novel multi-antenna impulse radio UWB transceiver for broadband high-throughput 4G WLANs”, published in IEEE communications letters, vol. 8, No 7, pages 419 to 421, July 2004, and is illustrated by FIG. FIG. 2.
As FIG. 2 shows, each sending antenna simultaneously sends two waveforms w1(t) and w2(t), wi(t) being a pulse corresponding to the ith Hermite function. One of these waveforms modulates the training sequence and the other is used to send the data. By simultaneously sending training and data sequences, this architecture is very different from the architecture of FIG. 1, where the training phase precedes the data sending phase.
The two known architectures that have just been briefly described can admittedly be used to increase the throughput of the sender, but to the detriment of the quality of the received signals, particularly because of the disturbances between signals being sent.